change background of transparent png to that of pdf

I created a transparent digital signature png from a scanned original handwritten signature.
It works fine for adding a signature to pdf documents with a white background.

For those times when the background is other shading/pattern, say a 'grayscale' looking shading/pattern...I want to 'adapt' the background of the signature.png to that of the pdf.

So in practice open the pdf, select an area that has different shading/pattern up to the clipboard, and make the background of the signature the same as that in clipboard; with hopes the background will 'blend' into the pdf.